Germanium-silicon thermoelement having fused tungsten contact



Aug. 29, 1967 I Q w HQRSTlNG 3,338,753

GERMANIUM-SILICON THERMOELEMENT HAVING FUSED' TUNGSTEN CONTACT OriginalFiled May 20, 1964 3 Sheets-Sheet 1 21 29 MIN-mil A/JYPE v I INVEN TOR.(AR 1 l4! flown/v BY g- 29, 1967 c. w. HORSTING 3,338,753

GERMANIUM-SILICON THERMOELEMEN' I HAVING FUSED TUNGSTEN CONTACT OriginalFiled May 20, 1964 5 Sheets-Sheet 2 mam/2v: 67419:: M warm/a 1967 w.HORSTING GERMANIUM-SILICON THERMOELEMENT HAVINGFUSED TUNGSTEN CONTACT 5Sheet$-$heet 3 Original Filed May 20, 1964 j I l a m .ym m m um. w /s M.w m w -x W -m by 0 a 0 0 M .4. w m m M m 9 United States Patent Ofltice3,338,753 GERMANlUM-SILICON THERMOELEMENT HAV- ING FUSED TUNGSTENCONTACT Carel W. Horsting, Caldwell, N.J., assignor to Radio Corporationof America, a corporation of Delaware Application May 20, 1964, Ser. No.370,395, now Patent No. 3,235,957, dated Feb. 22, 1966, which is acontinuation of application Ser. No. 143,446, Oct. 6, 1961. Divided andthis application July 12, 1965, Ser. No. 471,079

4 Claims. (Cl. 136-237) This application is a division of myapplication, Ser. No. 370,395, filed May 20, 1964, now US. Patent3,235,- 957, 'assigned to the assignee of this application; Ser. No.370,395 being a continuation of my application, Ser. No. 143,446, filedOct. 6, 1961, now abandoned.

This invention relates to improved thermoelectric devices utilizinggermanium-silicon alloy bodies with low sistance tungsten contacts.

Germanium-silicon alloys have been utilized for infrared detectordevices, as described in United States Patent 2,953,529, issued Sept.20, 1960, to M. L. Schultz and assigned to the same assignee as theinstant application; for semi-conductor devices, as described in UnitedStates Patent 2,817,798, issued Dec. 24, 1957 to D. A. Jenny andassigned to the same assignee as the instant application; and forthermoelectric devices, as described in application Ser. No. 229,830,now US. Patent No. 3,279,- 954, filed Nov. 11, 1962, and assigned to theassignee of this application. In these and other devices, it isfrequently necessary to make mechanically strong but low electricalresistance contacts to the germanium-silicon alloy bodies. Such contactshave been relatively diflicult and expensive to make by the methods ofthe prior art, and tend to exhibit low conductivity when theirmechanical strength is high, or low mechanical strength when theirconductivity is high. Part of the difficulty is that the thermalexpansion coeflicient of most contact materials is different from thatof germanium-silicon alloys.

It is therefore an object of the instant invention to provide improvedmaterials for making low resistance electrical contacts togermanium-silicon alloy bodies.

Another object of the invention is to provide improved materials forobtaining thermostable mechanically strong contacts to therrnoelementscomposed of germanium-silicon alloys.

Still another object of the invention is to provide germanium-siliconalloy bodies with contacts having about the same thermal coeflicient ofexpansion as the bodies.

Yet another object of the invention is to provide a low resistanceelectrical connection between a tungsten body and a thermoelectriccomponent which consists of germanium-silicon alloys.

' It has unexpectedly been found that a germaniumsilicon alloy body canbe bonded to a tungsten body by contacting the two bodies in anon-oxidizing ambient while applying heat. The bond thus formed betweenthe germanium-silicon alloy body and the tungsten body is inexpensive,easily fabricated, mechanically strong, unaffected by elevatedtemperatures, and exhibits a surprisingly low electrical resistance.

The invention and its advantages and features will be described ingreater detail by the following examples, in conjunction with theaccompanying drawing, in which:

FIG. 1 is a cross-sectional view of a germanium-silicon alloy body beingbonded to a tungsten body according to one embodiment of the invention;

FIG. 2 is a cross-sectional view of a germanium-silicon body in theprocess of being provided with a mechanically strong low-resistancebonded contact on each of two opposing faces according to anotherembodiment of the invention;

3,338,753 Patented Aug. 29, 1967 FIG. 3 is a cross-sectional view of athermoelectric Seebeck device according to the invention; I

FIG. 4 is a photomicrograph of a cross section of thIe bond area of abonded assembly such as that of F G. 1;

FIG. 5 is a graph showing the concentration of the various chemicalelements in a bond similar to that shown in FIG. 4; and

FIG. 6 is a graph containing the phase diagram for the binary systemsilicon-germanium.

Example I In this example, a germanium-silicon alloy body 10 iscontacted to a tungsten body 12 as illustrated in FIG. 1. Thegermanium-silicon alloy body 10 may be either polycrystalline ormonocrystalline. The exact composition of the germanium-silicon alloy isnot critical, and may, for example, consist of 25-50 atomic percentgermanium, balance (75-50 atomic percent) silicon. The germaniumsiliconbody 10 may be either intrinsic or extrinsic, p-type or n-type, lightlydoped or heavily doped. The exact size and shape of germanium siliconbody 10 is not critical. In this example, body 10 is in the form of awafer about inch square and about inch thick, and consists ofmonocrystalline n-type germanium-silicon alloy containing 25 atomicpercent germanium and 75 atomic percent silicon. The tungsten body 12 inthis example is of the same size and shape as the germanium-silicon body10. Preferably the mating surfaces of the two bodies are flat.

The germanium-silicon body 10 may be pressed against the tungsten body12 by any convenient method. Very little pressure is required. The upperlimit of the pressure that can be applied is that pressure which woulddeform the germanium-silicon body. Moderate pressures in the range ofabout 1 to 200 lbs. per square inch on the mating surfaces between thetwo bodies have been found satisfactory in practice. The pressure may beapplied in a convenient and simple manner by placing a weight 11 on thegermanium-silicon body 10 as shown in FIG. 1. A suitable material forweight 11 is stainless steel, since this material is not affected by thesubsequent heating step.

The assemblage of germanium-silicon body 10, weight 11 and tungsten body12 is then heated in a non-oxidizing ambient to a temperature of about10001100 C. The exact heating profile utilized does not appear to becritical in the practice of the invention. Heating of the assemblage forabout 30 minutes has been found satisfactory. The non-oxidizing ambientutilized may consist of a reducing gas such as hydrogen or forming gas(1 volume H and 9 volumes N or an inert gas such as argon. Non-oxidizingambients are utilized in order to prevent any undesirable side reactionssuch as oxidation of the germanium-silicon body. Such side reactions canalso be prevented by performing the heating step in a vacuum furnace atresidual atmospheric pressures of about 2 10 torr, since the amount ofoxygen remaining in the furnace atmosphere at this reduced pressure isinsufiicient to injure the germanium-silicon body by undesirable sidereactions. A vacuum may thus be regarded as a non-oxidizing ambient. Inthis example, the assemblage of weight 11, germanium-silicon body 10 andtungsten body 12 is heated in a furnace (not shown) to about 1100 C. inan atmosphere of non-oxidizing gas for about 30 minutes. The assemblageis then cooled to room temperature in the non-oxidizing ambient, andremoved from the furnace.

, andthermal resistance, and remains stable even after pro- 3 longedheating in vacuum at temperatures as high as 650 C.

Although the method of making the bond or joint between thegermanium-silicon body 14) and the tungsten body 12 was known by me, theexact nature of the bond itself was not known at the time my applicationSer. No. 143,446 was filed. However, microscopic examination of across-section of the joint area had shown a thin layer of new substancesor phases which had formed in the contact area between the tungsten body12 and the germanium-silicon body 10. This layer appeared to containelements of the originalmaterials, and was firmly joined to each of theoriginal bodies. The microscopic examinations showed this layer toconsist of two principal regions: (1) a distinctly delineated layer orzone A located directly next to the tungsten body 12; and (2) a lessdistinctly delineated layer or zone B between the distinct layer A andthe germanium-silicon body 10. The layer B showed indications of havingbeen partly molten. FIG. 4 is a photomicrograph of a polished crosssection of such a bond, clearly showing the tungsten body 12 on theleft, the germanium-silicon body 10 on the right, and the twointermediate layers A and B.

It is now definitely known that the bond between the bodies 10 and 12 isthe result of a chemical reaction. Detailed analysis has revealed thestructure of the bond and the nature of the process by which it isformed. Conventional X-ray analyses showed that the distinct layer Anext to the tungsten body is tungsten disilicide (WSi A more detailedanalysis has been made by the use of an electron beam probe method. Inthis method, the concentration of one or more elements is measured byfocusing a small beam of electrons on the spot to be analyzed andobserving the resultant X-radiation. By taking a series of suchsnapshots across the bond, for the three elements, silicon, germaniumand tungsten, a detailed picture was obtained of the chemicalcomposition of the bond. FIG. 5 shows the results of such an electronbeam probe analysis of a bond between a tungsten body 12 and a body of55% germanium and a 45% silicon (weight percent). The graph showncontains three curves, marked Si, Ge and W, showing the changes in theconcentration of the three elements as the bond is traversed. Tracingthe curves from right to left, corresponding to travel across the bondin FIG. 4 from left to right, starting in the tungsten body 12 andending in the Ge-Si alloy body 10, one observes the following:

(1) The tungsten curve starts at the 100% level in the pure tungstenshoe 12. At point C, it begins a sharp descent to about 75%, whichcorresponds to the tungsten concentration in WSl2, and is horizontalthrough the silicide layer A. At point D, corresponding to the interfacebetween the two intermediate layers A and B of the bond, the tungstencurve drops sharply to zero, showing that the second more diffuse layerB does not contain appreciable tungsten.

(2) The silicon curve starts with zero concentration on the right, andincreases sharply at point E, corresponding to point C, to about 25%silicon in the tungsten disilicide layer A. The curve is horizontalacross layer A, rises somewhat at point P, and then drops sharply almostto zero at the interface between layers A and B. Then, the siliconcontent remains low over the major portion of layer B, followed by agradual rise to about 25%, at point G where the probing wasdiscontinued.

(3) The germanium curve starts at zero at the interface between layers Aand B, climbs steeply to almost 100% at point H, is nearly horizontalover the major point of layer B, and then gradually drops to about 75%at point I, corresponding to point G.

The electron beam probe results shown in FIG. 5 not only confirm thepresence of the tungsten disilicide layer A but also explain the natureof the less clearly delineated layer B. It is evident that, in theformation of the bond, some of the tungsten of body 12 combines withsome of the silicon of alloy body 10 to form a layer of tungstendisilicide. But tungsten does not combine with germanium to formtungsten germanide. Therefore, since silicon is removed from thegermanium-silicon alloy in the reaction, the alloy layer B next to thesilicide layer A becomes enriched in germanium, sometimes to puregermanium.

It can be seen from the silicon-germanium phase diagram, shown in FIG.6, that the melting temperatures of silicon-germanium alloy compositionsdecrease with smaller percentages of silicon in the composition. Thisexplains the molten nature of the germanium rich layer B. That is, bydepleting the original silicon-germanium alloy composition of silicon,at least a portion of the resulting germanium-rich layer B becomesmolten or partially molten during the heating step.

Since the bond between the tungsten body 12 and alloy body 10 isproduced by a chemical reaction, it is necessary that the bodies be heldin contact at the reaction temperature. Any pressure that is sufficientto maintain contact, but below a value that would deform the alloy body,is satisfactory. Thus, the pressure may be as low as a fraction of apound per square inch or as high as several hundred p.s.i. Pressures of1 to 2 psi. are now being used satisfactorily. It appears that a bondwill be formed as soon as a complete layer of WSi is present, even if itis only of the order of 0.1 mil thick. However, a thickness of 1 to 2mils is preferred.

Example II If desired, a tungsten block or body may be bonded toopposite ends or faces of the germanium-silicon body, thus making aplurality of low-resistance contacts to the germanium-silicon body, asdescribed below.

In this example, the germanium-silicon body 20 (FIG.

-2) is disc shaped, polycrystalline, of p-type conductivity,

and consists of 50 atomic percent germanium50 atomic percent silicon.This corresponds to about 72.1 percent germanium and 27.9 percentsilicon, in weight percent. The germanium-silicon body is sandwichedbetween two tungsten bodies 22 and 24. Conveniently, tungsten bodies 22and 24 are discs of the same thickness and diameter as thegermanium-silicon body 20.

The tungstensemiconductor-tungsten sandwich thus assembled is placed ina suitable clamp or press 60. While more elaborate jigs may be utilized,if available, the simple differential expansion clamp 60 illustrated inFIG. 2 has been found satisfactory. This expansion clamp 60 comprisestwo thermal expansion members 23 and 25 which press against tungstenbodies 22 and 24, respectively. The two expansion members 23 and 25 areurged toward each other by steel cross bars 27 and 29, respectively.Cross bars 27 and 29 are held together by a pair of bolts 26 and 28. Anut 21 at each end of bolts 26 and 28 is used to adjust the pressureexerted by thermal expansion members 23 and 25 against thetungsten-semiconductortungsten sandwich. While stainless steel ispreferred for the thermal expansion members 23, 25 the remaining partsof this expansion clamp 60 may be made of ordinary steel.

The assemblage of the germanium-silicon body 20, the two tungsten bodies22 and 24, and the differential expansion clamp 60 hold-ing them is nextheated in a vacuum furnace (not shown) at a temperature of about 1000*C. for about 30 minutes. The residual atmospheric pressure within thefurnace is maintained at about 2X 10" torr. During the heating step thetwo stainless steel members 23 and 25 expand more than the steel rods26, 28 and thereby increase the pressure between the two tungsten bodies22 and 24. Pressures as high as necessary are thus easily attained.

The assemblage is next permitted to cool at room temperature and thenremoved from the furnace. When the tungsten-semiconductor-tungstensandwich is removed from the expansion clamp 60, it is found that thecomponents of the sandwich have been firmly joined together. The bondbetween each tungsten body 22 and 24 and the germanium-silicon body 20is mechanically strong, exhibits low thermal and electrical resistivityand is stable over prolonged periods of time despite repeated cycling invacuum to temperatures as high as 650 C.

Example III The method of the invention may also be utilized tofabricate thermoelectric devices, as described in the following example.

Thermoelectric devices for converting heat energy directly to electricalenergy by means of the Seebeck eifect generally comprise twothermoelectric bodies as thermo electric circuit members or components.The two thermoelectric bodies, also'known as thermoelements, are bondedat one end to a block of metal so as to form a thermoelectric junction.The two thermoelectric bodies are of opposite thermoelectric types, thatis, one thermoelement is made of P-type thermoelectric material and theother of N-type thermoelectric material. The designation of a particularthermoelectric material as N-type or P-type depends upon the directionof current flow across the cold junction of a thermocouple formed-by thethermoelectric material in question and a metal such as lead, when thethermocouple is operating as a thermoelectric generator according to theSeebeck effect. If the current direction in the external circuit ispositive toward the thermoelectric material then the material isdesignated P-type; if the current direction in the external circuit isnegative toward thethermoelectric material, then the material isdesignated as N-type. The present invention relates to both P-type andN-type thermoelectric germanium-silicon materials generally.

The two thermoelectric bodies should have a low electrical resistivity,since the Seebeck EMF generated in a device of this type is dependentupon the temperature difference between the hot and cold junctions ofthe device. The generation of Ioulean heat in a thermoelectric devicedue to the electrical resistance of either thermoelement, or to theresistance of the electrical contacts on either thermoelement, willreduce the efliciency of the device. The presence of high resistancecontacts on the thermoelements has been a serious problem in thefabrication of both Seebeck and Peltier thermoelectric devices. Highresistance contacts have reduced the cooling effect of Peltier devicesas much as 40% below the theoretical maximum value.

Referring to FIG. 3, the thermoelectric device 50 for the directconversion of thermal energy to electrical energy by means of theSeebeck effect comprises a P-type thermoelectric body of thermoelement30, and an N-type thermoelectric body or thermoelement 40. The twothermoelements 30 and 40 are conductively joined at one end to a metalplate 35. The other end of each of the thermoelements 30 and 40 isbonded to electrical contacts 32 and 42, respectively. Contacts 32 and42 are preferably metallic blocks or bodies to which electrical leadwires 34 and 44 respectively may be readily attached. For highestefficiency in the conversion of heat to electricity by the Seebeckeffect, the electrical resistance between each thermoelement (30 and 40)and metal plate 35, and the electrical resistance between eachthermoelement and its respective contact (32 and 42) should beminimized.-

In the operation of device 50, the metal plate 35 and its junctions tothe thermoelements 30 and 40 is heated to a temperature T and becomesthe hot junction of the device. The metal contacts 32 and 42 onthermoelements I 30 and 40, respectively, are maintained at atemperature 42, respectively. The electromotive force developed underthese conditions produces in the external circuit a flow of(conventional) current.(I) in the direction shown by arrows in FIG. 3;that is, the current flows in the external circuit from the P-typethermoelement 30 toward the N-type thermoelement 40.- The device isutilized by connecting a load R shown as a resistance 37 in the drawing,between the lea-d wires 34 and 44 which are attached to contacts 32 and42 of thermoelements 30 and 40, respectively.

The thermoelectric bodies of thermoelements 30 and 40 each may consistof a germanium-silicon alloy containing 25-50 atomic percent germanium.In this example, both of the two thermoelectric bodies 30 and 40*consist of polycrystalline germanium-silicon alloys containing 50 atomicpercent germanium. Thermoelement 30 contains an excess of acceptors soas to be P-type, while thermoelement 40* contains an access of donorsand hence is N-type. The metal plate 35, and the two metal bodies 32 and42 which are bonded to thermoelements 30 and 40, respectively and serveas low resistance contacts thereto, are all made of tungsten. Ifdesired, the tungsten contacts 32 and 42 may first be bonded to one endof the thermoelements 30 and 40, respectively in the manner described inExample 1 above, and then the other end of the thermoelements 30 and 40bonded to plate 35 in a second and subsequent operation. Alternatively,the plate 35, thermoelements 30 and 40.and contacts 32 and 42 may all bepositioned in a jig or clamp in a manner similar to that described inExample II above, and then the entire assemblage heated in a vacuumfurance or nonoxidizing ambient so as to bond or fuse the tungstenbodies (32, 35 and 42) to the germanium-silicon bodies (30 and 40) in asingle operation.

The Seebeck device 50 thus fabricated combines a number of importantadvantages. First, the thermoelectric device 50 can be operated atelevated temperatures. In the prior art, when solder was used to' bondthe thermoelements 30 and 40 to metal plate 35 and metal contacts 32 and42, the melting point of the solder was necessarily suificiently low soas not to injure-the thermoelements. Subsequently such prior artthermoelectric devices could not be operated at temperatures high enoughto soften the solder. This is a serious limitation, as thethermoelectric device 50' may be regarded as a heat engine, and hencefor a high Carnot efficiency requires a large temperature differencebetween the hot and cold junctions. Since the cold junction is generallyat room temperature, the hot junction temperature should be as high aspossible for maximum efiiciency. In the device 50 of this example, thereis no low-melting solder, and the tungsten bodies utilized as contactscan withstand very elevated temperatures. Hence the only limitation onthe hot junction temperature for'the device 50' is that imposed by themelting point of the enriched germanium zone B of the germanium-siliconalloy.

Second, the bonds or joints between the germaniumsilicon bodies (30 and40) and the tungsten bodies (32, 35 and 42) in the device 50 aremechanicaly very strong. A bond thus formed was not broken when shocktested under acceleration of g. The best bonds in the device 50 areobtained when the silicon-germanium ratio is chosen such that a goodmatch exists between the thermal coefiicient of expansion of thegermanium-silicon body and that of the tungsten bodies. Such a match isobtained with an alloy containing about 70 atomic percent silicon.

Third, the electricl resistance between the germaniumsilicon bodies orthermoelements (30 and 40) and the tungsten bodies (32, 35 and 42) ofthe device 50 is very low. The interface resistance between suchthermoelements and their tungsten contacts has been found too low tomeasure readily. As discussed above, such low resistance is veryimportant to optimize the efliciency of the device. I

Fourth, the bonds or joints between the germaniumsilicon bodies and thetungsten bodies in these thermoelectric devices are thermostable. Thedevices such as 50 of Example III can be utilized for prolonged periodsat elevated temperatures, or can be repeatedly cycled to elevatedtemperatures, provided the ambient of the device is non-oxidizing andthe coeflicients of thermal expansion are matched.

Fifth, the thermal resistance of the bonds or joints between thegermanium-silicon bodies and the tungsten bodies of devices such as 50is low. This feature of high thermal conductivity across the interfaceis desirable for optimization of the efliciency of the device.

It will be understood that the various embodiments described above areby way of example only and not limitation. Various modifications may bemade without departing from the spirit and scope of the invention. Forexample, other jigs and clamps may be utilized to press together agermanium-silicon body and a tungsten body. Other non-oxidizing ambientssuch as nitrogen and helium may be utilized during the heating step.

What is claimed is:

1. A thermoelectric device comprising a thermoelectric body ofgermanium-silicon alloy fused to a tungsten contact body by a lowresistance bond consisting essentially of layers of tungsten disilicideand a germanium-silicon alloy containing a higher percentage ofgermanium than said thermoelectric body.

2. A theromelectric device as in claim 1 wherein said thermoelectricbody contains at least 50 atomic percent silicon.

3. A low resistance bond between a germanium-silicon alloy body and atungsten body consisting essentially of, in order between said alloybody and said tungsten body, a zone of germanium-silicon alloycontaining a higher percentage of germanium than said alloy body, and azone of tungsten disilicide.

4. A low resistance bond as claim 3 wherein said germanium-silicon alloybody contains at least 50 atomic percent silicon.

References Cited UNITED STATES PATENTS 2,597,752 5/1952 Salisbury136-208 2,6465 36 7/ 1953 Benzer et al 29486 3,000,085 9/1961 Green29194 X 3,000,092 9/1961 Scuro 136 237 X 3,151,949 10/1964 Plust et a1.29195 3,164,892 1/1965 Lieberman et a1. 136239 3,178,271 4/1965 Maisselet al. 29195 FOREIGN PATENTS 609,035 9/ 1948 Great Britain.

OTHER REFERENCES Hansen, M. (ed.): Constitution of Binary Alloys, 2nded., McGraw-Hill, 1958, pp. 779, 1203 and 1204.

Levitas: Physical Review, vol. 99, No. 6 955, pp. 1810 14.

ALLEN B. CURTIS, Primary Examiner.

WINSTON A. DOUGLAS, Examiner.

A. M. BEKELMAN, Assistant Examiner.

1. A THERMOELECTRIC DEVICE COMPRISING A THERMOELECTRIC BODY OFGERMANIUM-SILICON ALLOY FUSED TO A TUNGSTEN CONTACT BODY BY A LOWRESISTANCE BOND CONSISTING ESSENTIALLY OF LAYERS OF TUNGSTEN DISILICIDEAND A GERMANIUM-SILICONALLOY CONTAINING A HIGER PERCENTAGE OF GERMANIUMTHAN SAID THERMOELECTRIC BODY.
 3. A LOW RESISTANCE BOND BETWEEN AGERMANIUM-SILICON ALLOY BODY AND A TUNGSTEN BODY CONSISTING ESSENTIALLYOF, IN ORDER BETWEEN SAID ALLOY BODY AND SAID TUNGSTEN BODY, A ZONE OFGERMANIUM-SILICON ALLOY CONTAINING A HIGHER PERCENTAGE OF GERMANIUM THANSAID ALLOY BODY, AND A ZONE OF TUNGSTEN DISILICIDE.